Chess Opening Theory/1. f4/1...e5/2. fxe5/2...d6/3. Nf3/3...dxe5/4. Nxe5/4...Bd6

4...Bd6
This attacks the knight. Since 5. d4? Bxe5 6. dxe5 Qxd1+ is uncomfortable for White, taking on double isolated e-pawns and forfeiting castling rights, while 5. Nc4?? Qh4+! 6. g3 Qxg3+!! 7. hxg3 Bxg3# is lost, and 5. Nd3? Qh4+ 6. Nf2 Bxh2 is equal, White is well advised to transpose to mainline theory with /5. Nf3/.